Myostatin (thing)

Myostatin is a growth factor that prevents muscles from growing too large. It regulates muscle size from early development onwards. Therefore if myostatin is absent, muscles are able to grow much larger.

A German boy is known to have a mutation which blocks production of myostatin. At almost age five, he has muscles twice the size of his peers.
No doubt this could be very useful for military applications, among others.

Now we need to find one for brain development, and get governments to give newborns these drugs depending on their vocation.